1. Field of the Invention
The present invention relates to a signal modulation method, and more particularly, to a run length limit (RLL) coding method for enhancing the recording density of digital recording media.
2. Description of the Related Art
There are various methods for modulating a signal, during the recording and reproduction of the signal used for digital optical recording media and magnetic recording media. Modified frequency modulation (FM) is a representative modulation method.
Such modulation methods are used for compact disks, first-generation magneto-optical disks, and second-generation re-recordable optical disks. The optical disk modulation method is either a peak detecting method, for detecting the position of a peak, or an edge detecting method for detecting the edge of a mark. Minimum and maximum time intervals between such peaks or edges are critical characteristic values. They determine the resolution which is related to the recording density of a reproduced signal and a jitter margin, and are expressed as the restriction RLL of the number of continuous `0`s. Other critical variables which determine the recording density are error propagation, whether the number of input bits is variable, and the ratio of the number of input bits to the number of output bits, namely, the conversion ratio.
RLL (2,7) is a conventional modulation method used for first-generation magneto-optical disks. It has a relatively small jitter margin to be used for the edge detecting method. RLL (1,7) is a method obtained by improving the conventional modulation method. With this method, a large jitter margin can be secured by reducing the size of the minimum mark, and by improving a capacity and the size of a window by 33%. However, the RLL (1,7) modulation method has a problem in that the recording density is relatively low for a given resolution, in the case of optical recording, in which the minimum time interval is 2T and 3T (the diffraction limit).